any form of getting some type of action, such as, making out, feeling things, or oral pleasure or sex between 2 people. "A hook up" is whats referred to as doing those things when your not going out with anyone

Guy:"yo, i hooked up with kristin last night!"
Guy 2:"Nice, what did you do?"
Guy1:"we only got to third"

Hooking up has come to define sexual relationships for most of today's teens and young women. It can mean anything from kissing and touching to oral sex or intercourse. Vagueness is its hallmark. "A girl can say, 'I hooked up with so-and-so,' and no one knows what she did. It protects you and makes you a player at the same time," -Aparicio (Sophomore at Boston University)

In Australia hooking up just mean kissing (with tongue. So basically you go to a party, mingle with some people and then u spot a cute guy/girl and start talking and flirting or watever and then you guys kiss. Thats hooking up. Usually isn't followed by anything after that night.

Casual sexual activity. Sex while not in a relationship with eachother. Similar to a "one-night stand," but it can happen repetedly. As long as the two people are not serious about eachother, and they have sex, it's hooking up.